Method of separating ions

ABSTRACT

The present disclosure relates generally to a method of separating ions according to their ion mobility, comprising (i) accumulating a first population of ions in a first region of an ion mobility separator, (ii) separating said first population of ions according to their ion mobility in said first region of said ion mobility separator, and (iii) accumulating a second population of ions in said first region of said ion mobility separator whilst said first population of ions are being separated according to their ion mobility in said ion mobility separator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/596,451, filed May 16, 2017, now U.S. Pat. No. 10,446,385, whichclaims priority from and the benefit of United Kingdom patentapplication no. 1608653.0, filed on May 17, 2016. The entire contents ofthese applications are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to methods of separating ions,for example according to mass or ion mobility, and ion separators suchas ion mobility separators or mass separators.

BACKGROUND

A single ion trap may be used upstream of an ion mobility separator inorder to improve the duty cycle when transferring successive populationsof ions into the ion mobility separator. Application of relatively highvoltage extraction field is typically required to ensure rapid transferof ions from the ion trap to the ion mobility separator with minimaldispersion. Application of such a high voltage field can cause heatingof the ions, resulting in undesired fragmentation (e.g., of labilecompounds). However, slower transfer of ions from the ion trap can leadto a lower duty cycle.

In addition, the ion cloud may not be driven far enough into the ionmobility separator, and may not experience the desired ion mobilityseparation force during subsequent operation. In this case ions can belost or remain trapped at the front region of the ion mobilityseparator.

US2003/0141446 (Blanchard) discloses an ion detecting apparatus andmethods.

WO 99/47912 (Spangler) discloses an ion mobility storage trap andmethod.

U.S. Pat. No. 7,838,826 (Park) discloses an apparatus and method forparallel flow ion mobility spectrometry combined with mass spectrometry.

WO 2014/140579 (Micromass) discloses a coaxial ion guide.

It is desired to provide an improved method of separating ions accordingto their ion mobility.

SUMMARY

According to an aspect of the present disclosure there is provided amethod of separating ions according to their ion mobility, comprising:

(i) accumulating a first or preceding population of ions in a firstregion of an ion mobility separator;

(ii) separating the first or preceding population of ions according totheir ion mobility in the first region of the ion mobility separator;and

(iii) accumulating a second or subsequent population of ions in thefirst region of the ion mobility separator whilst the first or precedingpopulation of ions are being separated according to their ion mobilityin the ion mobility separator.

The above method provides a first region of an ion mobility separatorthat acts as both an accumulation region and an ion mobility separationregion. Successive populations of ions may be accumulated in the firstregion and then separated according to their ion mobility in the firstregion. This avoids having to rapidly transfer successive populations ofions from an accumulation region (e.g., an external ion trap) into theion mobility separator to be separated, as they are already presentwithin it.

Conventional arrangements, for example those described in US2003/0141446(Blanchard), do not disclose an accumulation region that is also used asan ion mobility separation region.

The separation of the first population of ions may not be limited to thefirst region. As discussed herein, the ion mobility separation regionmay include the first region, as well as further ion mobility separationregions downstream thereof.

In various embodiments the first region may act in a trapping modeduring step (i) and then switch to an ion mobility separation mode instep (ii). The switching may be instantaneous or substantiallyinstantaneous.

Step (ii) may comprise accumulating the second population of ions in anaccumulation region upstream of the first region, whilst the firstpopulation of ions are being separated according to their ion mobilityin the ion mobility separator (e.g., in the first region). The upstreamaccumulation region may be or form part of an ion trap, for example anion trap external to the ion mobility separator.

The use of two accumulation regions has been found to improve uponarrangements that involve only a single accumulation region, for examplewhere an accumulating ion trap is located upstream of an ion mobilityseparator. This avoids, for example, transferring ions into the ionmobility separator (or ion mobility separation region) suddenly, whichcan lead to ion losses and/or fragmentation.

Step (iii) may comprise transferring the second population of ions fromthe upstream accumulation region to the first region. Step (iii) mayfurther comprise continuing to pass ions through the upstreamaccumulation region and into the first region, for example to add toand/or increase the number of ions in the second population of ions (nowaccumulating in the first region).

The ion mobility separator may comprise a second region downstream ofthe first region, and step (ii) may comprise separating the firstpopulation of ions according to their ion mobility in the first andsecond regions of the ion mobility separator. Upon initiation of step(ii), the first population of ions may all be located within the firstregion, and a driving force may be applied to drive the first populationof ions along the ion mobility separation region of the ion mobilityspectrometer (which may comprise the first and second regions).

Step (iii) may comprise accumulating the second population of ions inthe first region of the ion mobility separator whilst the firstpopulation of ions are being separated according to their ion mobilityin the second region of the ion mobility separator.

Step (iii) may comprise accumulating the second population of ions inthe first region of the ion mobility separator after the firstpopulation of ions have exited the first region of the ion mobilityseparator.

The method may further comprise:

(iv) separating the second population of ions according to their ionmobility in the ion mobility separator; and

(v) accumulating a third population of ions in the first region of theion mobility separator whilst the second population of ions are beingseparated according to their ion mobility.

An incoming beam of ions, for example a continuous beam of ions, may bedirected towards the upstream accumulation region.

Taking the second population of ions as an example, this population maybegin to accumulate in the upstream accumulation region with ions fromthe incoming beam of ions.

Once the first population of ions has left the first region of the ionmobility separator, the ions within the upstream accumulation region(i.e., the second population of ions at that point) are transferred tothe first region. This period may be relatively slow to avoid ionheating and undesired fragmentation.

During this period ions from the incoming beam of ions may continue toenter the upstream accumulation region and pass through to the firstregion, adding to and increasing the number of ions in the secondpopulation of ions, which is now accumulating in the first region of theion mobility separator.

It will be appreciated that, throughout this cycle, ions from theincoming beam of ions may either accumulate in the upstream accumulationregion, or pass through this region and accumulate in the first regionof the ion mobility separator. This can mean that no ions from theincoming beam of ions are lost to the system or otherwise, and a 100%duty cycle can be achieved.

The method may further comprise:

repeating steps (ii) to (v) for the third and further populations ofions, such that subsequent populations of ions are accumulated in thefirst region of the ion mobility separator whilst preceding populationsof ions are being separated according to their ion mobility.

Step (v) may comprise accumulating the third population of ions in theupstream accumulation region whilst the second population of ions arebeing separated according to their ion mobility in the ion mobilityseparator.

Step (iv) may comprise separating the second population of ionsaccording to their ion mobility in the first region of the ion mobilityseparator, or step (iv) may comprise separating the second population ofions according to their ion mobility in the first and second regions ofthe ion mobility separator.

Step (v) may comprise accumulating the third population of ions in thefirst region of the ion mobility separator whilst the second populationof ions are being separated according to their ion mobility in thesecond region of the ion mobility separator.

Step (v) may comprise accumulating the third population of ions in thefirst region of the ion mobility separator after the second populationof ions have exited the first region of the ion mobility separator.

Any or all of the steps (i) to (v) may occur sequentially in time.

During a first time period T₀-T₁ the first population of ions may beaccumulated in the first region.

During a second time period T₁-T₂ the first population of ions may beseparated according to their ion mobility (e.g., in the ion mobilityseparation region of the ion mobility separator, which may comprise thefirst region and the second region), and the second population of ionsmay be accumulating in the upstream accumulation region.

During a third time period T₂-T₃ the first population of ions may haveexited the first region, but may not have exited the second region, andthe second population of ions may now be transferred to and beginaccumulating in the first region. The second population of ions mayinclude ions that enter and are passed through the upstream accumulationregion and reach the first region of the ion mobility separator duringthis period.

During a fourth time period T₃-T₄, the second population of ions mayfinish accumulating in the first region, and may be separated accordingto their ion mobility (e.g., in the ion mobility separation region), andthe third population of ions may be accumulating in the upstreamaccumulation region. The first population of ions may have exited theion mobility separator or ion mobility separation region.

During a fifth time period T₄-T₅, the second population of ions may haveexited the first region, but may not have exited the second region, andthe third population of ions may now be transferred to and beginaccumulating in the first region.' The third population of ions mayinclude ions that enter and are passed through the upstream accumulationregion and reach the first region of the ion mobility separator duringthis period.

During a sixth time period T₅-T₆, the third population of ions may beseparated according to their ion mobility (e.g., in the ion mobilityseparation region), and a further population of ions may be accumulatedin the upstream accumulation region. The second population of ions mayhave exited the ion mobility separator or ion mobility separationregion.

It will be appreciated that this cycle could continue for furtherpopulations of ions.

The ion mobility separator may comprise an RF-confined ion mobilityseparator.

The ion mobility separator may comprise a plurality of electrodes oraxial groups of electrodes stacked adjacent to one another, whereinalternate phases of an RF voltage are applied to adjacent electrodes orelectrodes within the axial groups of electrodes.

Each of the plurality of electrodes may comprise apertures through whichions travel in use.

The plurality of electrodes may comprise a plurality of pairs ofelectrodes stacked adjacent to one another, wherein alternate phases ofan RF voltage are applied to adjacent pairs of electrodes.

Each pair of electrodes may comprise a first plate electrode opposite asecond plate electrode, wherein in use ions travel through the gapbetween the first and second plate electrodes.

The first region and/or the second region of the ion mobility separatormay comprise RF-confined regions of the ion mobility separator. The ionmobility separator may comprise a longitudinal stack of electrodes, oraxial groups of electrodes, and the first region and/or the secondregion may comprise portions or sections of said longitudinal stack ofelectrodes, or axial groups of electrodes.

The ion mobility separator may be located within a single region orvacuum region, for example of a mass or ion mobility spectrometer.

The first region of the ion mobility separator may be arranged andadapted to switch between a trapping mode, in which ions aresubstantially trapped within the first region, and an ion mobilityseparation mode, in which ions are separated according to their ionmobility within the first region.

The upstream accumulation region may be arranged and adapted to trap oraccumulate a population of ions and then transfer the population of ionsfrom the sec upstream accumulation region into the first region.

The upstream accumulation region may be arranged and adapted to switchbetween a trapping mode, in which ions are substantially trapped withinthe upstream accumulation region, and an ion guiding mode, in which ionsare passed from and/or through the upstream accumulation region into thefirst region.

The upstream accumulation region may comprise RF-confined regions of anion trap. The ion trap may comprise a longitudinal stack of electrodes,or axial groups of electrodes, and the upstream accumulation region maycomprise portions or sections of said longitudinal stack of electrodes,or axial groups of electrodes.

The populations of ions, for example any of the first, second or thirdpopulations of ions, may be accumulated in the first region such thatwithin the first region the first population of ions has a relativelysmall spatial spread in the direction of ion mobility separation, and arelatively large spatial spread in the direction orthogonal to ionmobility separation. The direction of ion mobility separation may bedefined as the direction of ion movement when ions are separatedaccording to their ion mobility within the ion mobility separator, forexample during steps (ii) and/or (iv).

A control system of the ion mobility separator may be arranged andadapted to cause ions to accumulate in the first region such that,within the first region, ions have a relatively small spatial spread inthe direction of ion mobility separation, and a relatively large spatialspread in the direction orthogonal to ion mobility separation, forexample immediately prior to the ions being separated according to theirion mobility.

In various embodiments the method may instead be a method of separatingions according to mass or mass to charge ratio, and the ion mobilityseparator may instead be a mass separation device, for example ahigh-pressure mass separation device. The features described above inrelation to the ion mobility separator, for example the arrangement ofelectrodes and/or the voltages applied thereto, may also be features ofthe mass separation device. The method features relating to the first,second and accumulation regions may be applied mutatis mutandis to amass separation device comprising these regions and arranged in the samemanner as the ion mobility separator described above. For example, insuch embodiments the first region may be arranged and adapted to switchbetween a trapping mode, in which ions are substantially trapped withinthe first region, and a separation mode, in which ions are separatedaccording to mass or mass to charge ratio within the first region.

According to an aspect of the present disclosure there is provided anion mobility separator comprising a first region arranged and adapted toaccumulate consecutive populations of ions, and a control systemarranged and adapted:

(i) to accumulate a first or preceding population of ions in the firstregion of the ion mobility separator;

(ii) to separate the first or preceding population of ions according totheir ion mobility in the first region of the ion mobility separator;and

(iii) to accumulate a second or subsequent population of ions in thefirst region of the ion mobility separator whilst the first or precedingpopulation of ions are being separated according to their ion mobilityin the ion mobility separator.

According to an aspect of the present disclosure there is provided amethod of separating ions in a device, for example an ion mobilityseparation device, wherein the device comprises a first region, anaccumulation region upstream of the first region and a second regiondownstream of the first region, and the method comprises the steps of:

(i) driving a first population of ions through the first and secondregions so as to separate a first population of ions, for exampleaccording to their ion mobility, whilst accumulating a second populationof ions in the accumulation region;

(ii) after the first population of ions has exited the first region, butprior to the first population of ions exiting the second region,transferring the second population of ions into the first region andtrapping the second population of ions within the first region; and then

(iii) driving the second population of ions through the first and secondregions so as to separate the second population of ions, for exampleaccording to their ion mobility, whilst accumulating a third populationof ions in the accumulation region; and optionally

(iv) repeating steps (ii) and (iii) for the third and furtherpopulations of ions so as to successively separate the third and furtherpopulations of ions, for example according to their ion mobility.

The device may comprise a device for separating ions according to theirmass or mass to charge ratio, for example a high-pressure massseparation device, or a device for separating ions according to theirion mobility, for example an ion mobility spectrometer.

The first and second regions and/or the accumulation region may compriseRF-confined regions, for example an RF-confined regions of a massseparation device or an ion mobility separator.

The first and second regions and/or the accumulation region may comprisea plurality of electrodes or axial groups of electrodes stacked adjacentto one another, wherein alternate phases of an RF voltage are applied toadjacent electrodes or electrodes within the axial groups of electrodes.

Each of the plurality of electrodes may comprise apertures through whichions travel in use.

The plurality of electrodes may comprise a plurality of pairs ofelectrodes stacked adjacent to one another, wherein alternate phases ofan RF voltage are applied to adjacent pairs of electrodes.

Each pair of electrodes may comprise a first plate electrode opposite asecond plate electrode, wherein in use ions travel through the gapbetween the first and second plate electrodes.

The first region and/or the second region of the device may compriseRF-confined regions of the device. The device may comprise alongitudinal stack of electrodes, or axial groups of electrodes, and thefirst region and/or the second region and/or the accumulation region maycomprise portions or sections of said longitudinal stack of electrodes,or axial groups of electrodes.

The first region of the device may be arranged and adapted to switchbetween a trapping mode, in which ions are substantially trapped withinthe first region, and a separation mode, in which ions are separated,for example according to their mass, mass to charge ratio or ionmobility within the first region.

The accumulation region of the device may be arranged and adapted totrap or accumulate a population of ions and then transfer the populationof ions from the accumulation region into the first region.

The accumulation region of the device may be arranged and adapted toswitch between a trapping mode, in which ions are substantially trappedwithin the accumulation region, and an ion guiding mode, in which ionsare passed from and/or through the accumulation region into the firstregion.

According to an aspect of the present disclosure there is provided anapparatus comprising a device for separating ions, for example an ionmobility separation device, and a control system, wherein the devicecomprises a first region, an accumulation region upstream of the firstregion and a second region downstream of the first region, and thecontrol system is arranged and adapted:

(i) to drive a first population of ions through the first and secondregions so as to separate the first population of ions, for exampleaccording to their ion mobility, whilst accumulating a second populationof ions in the accumulation region;

(ii) after the first population of ions has exited the first region, butprior to the first population of ions exiting the second region, totransfer the second population of ions into the first region and trapthe second population of ions within the first region; and then

(iii) to drive the second population of ions through the first andsecond regions so as to separate the second population of ions, forexample according to their ion mobility, whilst accumulating a thirdpopulation of ions in the accumulation region; and optionally

(iv) to repeat steps (ii) and (iii) for the third and furtherpopulations of ions so as to successively separate the third and furtherpopulations of ions, for example according to their ion mobility.

According to an aspect of the present disclosure there is provided anion mobility separation device, for example a high duty cycle ionmobility separation device, the device comprising:

an RF confined ion mobility separation region;

a first ion trapping region upstream of said ion mobility separationregion; and

a second ion trapping region upstream of said first ion trapping region;

wherein in operation:

ions are accumulated in said first trapping region for a first portionof an ion mobility separation cycle time T₁-T₂; and

ions are accumulated in said second trapping region for a second portionof an ion mobility separation cycle time T₃-T₄.

The first trapping region may form part of the ion mobility separationregion for part of the ion mobility separation cycle.

The spectrometer may comprise an ion source selected from the groupconsisting of: (i) an Electrospray ionisation (“ESI”) ion source; (ii)an Atmospheric Pressure Photo Ionisation (“APPI”) ion source; (iii) anAtmospheric Pressure Chemical Ionisation (“APCI”) ion source; (iv) aMatrix Assisted Laser Desorption Ionisation (“MALDI”) ion source; (v) aLaser Desorption Ionisation (“LDI”) ion source; (vi) an AtmosphericPressure Ionisation (“API”) ion source; (vii) a Desorption Ionisation onSilicon (“DIOS”) ion source; (viii) an Electron Impact (“EI”) ionsource; (ix) a Chemical Ionisation (“CI”) ion source; (x) a FieldIonisation (“FI”) ion source; (xi) a Field Desorption (“FD”) ion source;(xii) an Inductively Coupled Plasma (“ICP”) ion source; (xiii) a FastAtom Bombardment (“FAB”) ion source; (xiv) a Liquid Secondary Ion MassSpectrometry (“LSIMS”) ion source; (xv) a Desorption ElectrosprayIonisation (“DESI”) ion source; (xvi) a Nickel-63 radioactive ionsource; (xvii) an Atmospheric Pressure Matrix Assisted Laser DesorptionIonisation ion source; (xviii) a Thermospray ion source; (xix) anAtmospheric Sampling Glow Discharge Ionisation (“ASGDI”) ion source;(xx) a Glow Discharge (“GD”) ion source; (xxi) an Impactor ion source;(xxii) a Direct Analysis in Real Time (“DART”) ion source; (xxiii) aLaserspray Ionisation (“LSI”) ion source; (xxiv) a Sonicspray Ionisation(“SSI”) ion source; (xxv) a Matrix Assisted Inlet Ionisation (“MAII”)ion source; (xxvi) a Solvent Assisted Inlet Ionisation (“SAII”) ionsource; (xxvii) a Desorption Electrospray Ionisation (“DESI”) ionsource; (xxviii) a Laser Ablation Electrospray Ionisation (“LAESI”) ionsource; and (xxix) Surface Assisted Laser Desorption Ionisation(“SALDI”).

The spectrometer may comprise one or more continuous or pulsed ionsources.

The spectrometer may comprise one or more ion guides.

The spectrometer may comprise one or more ion mobility separationdevices and/or one or more Field Asymmetric Ion Mobility Spectrometerdevices.

The spectrometer may comprise one or more ion traps or one or more iontrapping regions.

The spectrometer may comprise one or more collision, fragmentation orreaction cells selected from the group consisting of: (i) a CollisionalInduced Dissociation (“CID”) fragmentation device; (ii) a SurfaceInduced Dissociation (“SID”) fragmentation device; (iii) an ElectronTransfer Dissociation (“ETD”) fragmentation device; (iv) an ElectronCapture Dissociation (“ECD”) fragmentation device; (v) an ElectronCollision or Impact Dissociation fragmentation device; (vi) a PhotoInduced Dissociation (“PID”) fragmentation device; (vii) a Laser InducedDissociation fragmentation device; (viii) an infrared radiation induceddissociation device; (ix) an ultraviolet radiation induced dissociationdevice; (x) a nozzle-skimmer interface fragmentation device; (xi) anin-source fragmentation device; (xii) an in-source Collision InducedDissociation fragmentation device; (xiii) a thermal or temperaturesource fragmentation device; (xiv) an electric field inducedfragmentation device; (xv) a magnetic field induced fragmentationdevice; (xvi) an enzyme digestion or enzyme degradation fragmentationdevice; (xvii) an ion-ion reaction fragmentation device; (xviii) anion-molecule reaction fragmentation device; (xix) an ion-atom reactionfragmentation device; (xx) an ion-metastable ion reaction fragmentationdevice; (xxi) an ion-metastable molecule reaction fragmentation device;(xxii) an ion-metastable atom reaction fragmentation device; (xxiii) anion-ion reaction device for reacting ions to form adduct or productions; (xxiv) an ion-molecule reaction device for reacting ions to formadduct or product ions; (xxv) an ion-atom reaction device for reactingions to form adduct or product ions; (xxvi) an ion-metastable ionreaction device for reacting ions to form adduct or product ions;(xxvii) an ion-metastable molecule reaction device for reacting ions toform adduct or product ions; (xxviii) an ion-metastable atom reactiondevice for reacting ions to form adduct or product ions; and (xxix) anElectron Ionisation Dissociation (“EID”) fragmentation device.

The spectrometer may comprise a mass analyser selected from the groupconsisting of: (i) a quadrupole mass analyser; (ii) a 2D or linearquadrupole mass analyser; (iii) a Paul or 3D quadrupole mass analyser;(iv) a Penning trap mass analyser; (v) an ion trap mass analyser; (vi) amagnetic sector mass analyser; (vii) Ion Cyclotron Resonance (“ICR”)mass analyser; (viii) a Fourier Transform Ion Cyclotron Resonance(“FTICR”) mass analyser; (ix) an electrostatic mass analyser arranged togenerate an electrostatic field having a quadro-logarithmic potentialdistribution; (x) a Fourier Transform electrostatic mass analyser; (xi)a Fourier Transform mass analyser; (xii) a Time of Flight mass analyser;(xiii) an orthogonal acceleration Time of Flight mass analyser; and(xiv) a linear acceleration Time of Flight mass analyser.

The spectrometer may comprise one or more energy analysers orelectrostatic energy analysers.

The spectrometer may comprise one or more ion detectors.

The spectrometer may comprise one or more mass filters selected from thegroup consisting of: (i) a quadrupole mass filter; (ii) a 2D or linearquadrupole ion trap; (iii) a Paul or 3D quadrupole ion trap; (iv) aPenning ion trap; (v) an ion trap; (vi) a magnetic sector mass filter;(vii) a Time of Flight mass filter; and (viii) a Wien filter.

The spectrometer may comprise a device or ion gate for pulsing ions;and/or a device for converting a substantially continuous ion beam intoa pulsed ion beam.

The spectrometer may comprise a C-trap and a mass analyser comprising anouter barrel-like electrode and a coaxial inner spindle-like electrodethat form an electrostatic field with a quadro-logarithmic potentialdistribution, wherein in a first mode of operation ions are transmittedto the C-trap and are then injected into the mass analyser and whereinin a second mode of operation ions are transmitted to the C-trap andthen to a collision cell or Electron Transfer Dissociation devicewherein at least some ions are fragmented into fragment ions, andwherein the fragment ions are then transmitted to the C-trap beforebeing injected into the mass analyser.

The spectrometer may comprise a stacked ring ion guide comprising aplurality of electrodes each having an aperture through which ions aretransmitted in use and wherein the spacing of the electrodes increasesalong the length of the ion path, and wherein the apertures in theelectrodes in an upstream section of the ion guide have a first diameterand wherein the apertures in the electrodes in a downstream section ofthe ion guide have a second diameter which is smaller than the firstdiameter, and wherein opposite phases of an AC or RF voltage areapplied, in use, to successive electrodes.

The spectrometer may comprise a device arranged and adapted to supply anAC or RF voltage to the electrodes. The AC or RF voltage optionally hasan amplitude selected from the group consisting of: (i) about <50 V peakto peak; (ii) about 50-100 V peak to peak; (iii) about 100-150 V peak topeak; (iv) about 150-200 V peak to peak; (v) about 200-250 V peak topeak; (vi) about 250-300 V peak to peak; (vii) about 300-350 V peak topeak; (viii) about 350-400 V peak to peak; (ix) about 400-450 V peak topeak; (x) about 450-500 V peak to peak; and (xi)>about 500 V peak topeak.

The AC or RF voltage may have a frequency selected from the groupconsisting of: (i)<about 100 kHz; (ii) about 100-200 kHz; (iii) about200-300 kHz; (iv) about 300-400 kHz; (v) about 400-500 kHz; (vi) about0.5-1.0 MHz; (vii) about 1.0-1.5 MHz; (viii) about 1.5-2.0 MHz; (ix)about 2.0-2.5 MHz; (x) about 2.5-3.0 MHz; (xi) about 3.0-3.5 MHz; (xii)about 3.5-4.0 MHz; (xiii) about 4.0-4.5 MHz; (xiv) about 4.5-5.0 MHz;(xv) about 5.0-5.5 MHz; (xvi) about 5.5-6.0 MHz; (xvii) about 6.0-6.5MHz; (xviii) about 6.5-7.0 MHz; (xix) about 7.0-7.5 MHz; (xx) about7.5-8.0 MHz; (xxi) about 8.0-8.5 MHz; (xxii) about 8.5-9.0 MHz; (xxiii)about 9.0-9.5 MHz; (xxiv) about 9.5-10.0 MHz; and (xxv)>about 10.0 MHz.

The spectrometer may comprise a chromatography or other separationdevice upstream of an ion source. The chromatography separation devicemay comprise a liquid chromatography or gas chromatography device.Alternatively, the separation device may comprise: (i) a CapillaryElectrophoresis (“CE”) separation device; (ii) a CapillaryElectrochromatography (“CEC”) separation device; (iii) a substantiallyrigid ceramic-based multilayer microfluidic substrate (“ceramic tile”)separation device; or (iv) a supercritical fluid chromatographyseparation device.

The ion guide may be maintained at a pressure selected from the groupconsisting of: (i)<about 0.0001 mbar; (ii) about 0.0001-0.001 mbar;(iii) about 0.001-0.01 mbar; (iv) about 0.01-0.1 mbar; (v) about 0.1-1mbar; (vi) about 1-10 mbar; (vii) about 10-100 mbar; (viii) about100-1000 mbar; and (ix)>about 1000 mbar.

Analyte ions may be subjected to Electron Transfer Dissociation (“ETD”)fragmentation in an Electron Transfer Dissociation fragmentation device.Analyte ions may be caused to interact with ETD reagent ions within anion guide or fragmentation device.

Optionally, in order to effect Electron Transfer Dissociation either:(a) analyte ions are fragmented or are induced to dissociate and formproduct or fragment ions upon interacting with reagent ions; and/or (b)electrons are transferred from one or more reagent anions or negativelycharged ions to one or more multiply charged analyte cations orpositively charged ions whereupon at least some of the multiply chargedanalyte cations or positively charged ions are induced to dissociate andform product or fragment ions; and/or (c) analyte ions are fragmented orare induced to dissociate and form product or fragment ions uponinteracting with neutral reagent gas molecules or atoms or a non-ionicreagent gas; and/or (d) electrons are transferred from one or moreneutral, non-ionic or uncharged basic gases or vapours to one or moremultiply charged analyte cations or positively charged ions whereupon atleast some of the multiply charged analyte cations or positively chargedions are induced to dissociate and form product or fragment ions; and/or(e) electrons are transferred from one or more neutral, non-ionic oruncharged superbase reagent gases or vapours to one or more multiplycharged analyte cations or positively charged ions whereupon at leastsome of the multiply charge analyte cations or positively charged ionsare induced to dissociate and form product or fragment ions; and/or (f)electrons are transferred from one or more neutral, non-ionic oruncharged alkali metal gases or vapours to one or more multiply chargedanalyte cations or positively charged ions whereupon at least some ofthe multiply charged analyte cations or positively charged ions areinduced to dissociate and form product or fragment ions; and/or (g)electrons are transferred from one or more neutral, non-ionic oruncharged gases, vapours or atoms to one or more multiply chargedanalyte cations or positively charged ions whereupon at least some ofthe multiply charged analyte cations or positively charged ions areinduced to dissociate and form product or fragment ions, wherein the oneor more neutral, non-ionic or uncharged gases, vapours or atoms areselected from the group consisting of: (i) sodium vapour or atoms; (ii)lithium vapour or atoms; (iii) potassium vapour or atoms; (iv) rubidiumvapour or atoms; (v) caesium vapour or atoms; (vi) francium vapour oratoms; (vii) C₆₀ vapour or atoms; and (viii) magnesium vapour or atoms.

The multiply charged analyte cations or positively charged ions maycomprise peptides, polypeptides, proteins or biomolecules.

Optionally, in order to effect Electron Transfer Dissociation: (a) thereagent anions or negatively charged ions are derived from apolyaromatic hydrocarbon or a substituted polyaromatic hydrocarbon;and/or (b) the reagent anions or negatively charged ions are derivedfrom the group consisting of: (i) anthracene; (ii) 9,10diphenyl-anthracene; (iii) naphthalene; (iv) fluorine; (v) phenanthrene;(vi) pyrene; (vii) fluoranthene; (viii) chrysene; (ix) triphenylene; (x)perylene; (xi) acridine; (xii) 2,2′ dipyridyl; (xiii) 2,2′ biquinoline;(xiv) 9-anthracenecarbonitrile; (xv) dibenzothiophene; (xvi)1,10′-phenanthroline; (xvii) 9′ anthracenecarbonitrile; and (xviii)anthraquinone; and/or (c) the reagent ions or negatively charged ionscomprise azobenzene anions or azobenzene radical anions.

The process of Electron Transfer Dissociation fragmentation may compriseinteracting analyte ions with reagent ions, wherein the reagent ionscomprise dicyanobenzene, 4-nitrotoluene or azulene.

A chromatography detector may be provided, wherein the chromatographydetector comprises either:

a destructive chromatography detector optionally selected from the groupconsisting of (i) a Flame Ionization Detector (FID); (ii) anaerosol-based detector or Nano Quantity Analyte Detector (NQAD); (iii) aFlame Photometric Detector (FPD); (iv) an Atomic-Emission Detector(AED); (v) a Nitrogen Phosphorus Detector (NPD); and (vi) an EvaporativeLight Scattering Detector (ELSD); or

a non-destructive chromatography detector optionally selected from thegroup consisting of: (i) a fixed or variable wavelength UV detector;(ii) a Thermal Conductivity Detector (TCD); (iii) a fluorescencedetector; (iv) an Electron Capture Detector (ECD); (v) a conductivitymonitor; (vi) a Photoionization Detector (PID); (vii) a Refractive IndexDetector (RID); (viii) a radio flow detector; and (ix) a chiraldetector.

The spectrometer may be operated in various modes of operation includinga mass spectrometry (“MS”) mode of operation; a tandem mass spectrometry(“MS/MS”) mode of operation; a mode of operation in which parent orprecursor ions are alternatively fragmented or reacted so as to producefragment or product ions, and not fragmented or reacted or fragmented orreacted to a lesser degree; a Multiple Reaction Monitoring (“MRM”) modeof operation; a Data Dependent Analysis (“DDA”) mode of operation; aData Independent Analysis (“DIA”) mode of operation a Quantificationmode of operation or an Ion Mobility Spectrometry (“IMS”) mode ofoperation.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments will now be described, by way of example only, andwith reference to the accompanying drawings in which:

FIG. 1 shows a known arrangement in which an ion trap is positionedupstream of an ion mobility separator;

FIG. 2 shows an embodiment in accordance with the present disclosurewherein an accumulation and/or trapping region is provided within an ionmobility separator; and

FIG. 3 shows an embodiment wherein an ion mobility separator comprises aplurality of electrodes stacked adjacent to one another.

DETAILED DESCRIPTION

The present disclosure may relate generally to methods of separatingions, for example according to their ion mobility, in which a separationdevice may comprise a region (e.g., the first region referred to herein)that may be arranged and adapted to switch between a trapping mode, inwhich ions are substantially trapped within the region, and a separationmode, in which ions are separated, for example according to their massor ion mobility within the region.

The method may comprise (i) accumulating a first population of ions in afirst region of a device, such as an ion mobility separator or massseparation device, (ii) separating said first population of ionsaccording to their mass, mass to charge ratio or ion mobility in saiddevice, and (iii) accumulating a second population of ions in said firstregion of said device whilst said first population of ions are beingseparated according to their mass, mass to charge ratio or ion mobilityin said device. In various embodiments the device is an ion mobilityseparator, but may instead be a mass separation device such as a highpressure mass separation device.

This can provide better performance during separation since thepopulation of ions to be separated are already contained within the ionmobility separator, and do not have to be transferred into it rapidlyfrom e.g., an external ion trap. Ion heating and undesired fragmentationis avoided.

A known embodiment employing such an external ion trap will now bedescribed with reference to FIG. 1.

FIG. 1 shows an arrangement of an ion mobility spectrometer, wherein theion mobility spectrometer may comprise an ion trap 1 and an ion mobilityseparator 2. As will be appreciated, FIG. 1 shows the location of apopulation of ions schematically at three consecutive points in time,T₀-T₁, T₁-T₂, and T₂-T₃.

The time period T₀-T₁ represents the time during a first ion mobilityseparation cycle, wherein during this time period the ion trap 1 may befilled with a first population of ions. During the time period T₀-T₁ aseparate population of ions (not shown) may be separated within the ionmobility separator 2.

Over a subsequent time period T₁-T₂ the first population of ions may bedriven or transferred from the ion trap 1 and into the ion mobilityseparator 2. During the time period T₁-T₂ there may be no driving force(e.g., a DC voltage or travelling wave) applied to the ion mobilityseparator 2 that would cause the first population of ions to separateaccording to their ion mobility.

During the time period T₂-T₃ a driving force (e.g., a DC voltage ortravelling wave) may then be applied to ion mobility separator 2 so asto cause the first population of ions to be separated within the ionmobility separator 2 according to their ion mobility. During this periodT₂-T₃ a second population of ions may be accumulated in ion trap 1.

In general the time period T₀-T₁ may be equal to the time period T₂-T₃,in order to maintain high duty cycle.

The time period T₁-T₂ is usually kept relatively short. This is toensure that the populations of ions to be separated within the ionmobility separator 2 maintain a low spatial distribution in thedirection of separation, and so that the ion trap 1 is rapidly availablefor the accumulation of the second (or subsequent) population of ions.During this period ions that may otherwise have entered ion trap 1 maybe lost, which reduces the duty cycle of the ion mobility spectrometer.

The rapid ejection from the ion trap 1 into the ion mobility separator 2typically requires a high driving force, which can lead to fragmentationand ion losses as discussed in the background section above.

FIG. 2 shows an embodiment of the present disclosure, showing an ionmobility spectrometer or separation device comprising an ion mobilityseparator 10 and an upstream accumulation region 14. The ion mobilityseparator 10 comprises a plurality of regions 12, 16. The ion mobilityseparator 10 may not be limited to the regions shown, and more may beprovided if necessary.

A first region 12 of the ion mobility separator 10 may be arranged andadapted to switch between an ion trapping or accumulating mode, in whichions are substantially trapped or accumulated within the first region12, and an ion mobility separation mode, in which ions are separatedaccording to their ion mobility within the first region 12.

The accumulation region 14 may be upstream of the first region 12 andmay be arranged and adapted to trap or accumulate a population of ionsin an ion trapping or accumulating mode, and then transfer ions from theaccumulation region 14 into the ion mobility separator 10, specificallythe first region 12 thereof, in an ion guiding mode, wherein ions arepassed through and/or from the accumulation region 14 into the firstregion 12.

Ions may continuously enter the accumulation region 14 (e.g., from acontinuous source of ions). The incoming ions may pass through theaccumulation region 14 when it is operated in the ion guiding mode, andbe stored in the accumulation region 14 when it is operated in the iontrapping mode.

A second region 16 may be downstream of the first region 12 (and theaccumulation region 14) and may be arranged and adapted to receive ionsfrom said first region 12 and separate them according to their ionmobility.

Thus, the arrangement shown in FIG. 2 comprises two trapping regions,namely the first region 12 and the accumulation region 14. The firstregion 12 is part of the ion mobility separator 10 and may operate inboth a trapping mode and an ion mobility separation mode. Theaccumulation region 14 may be a separate ion trap. The first region 12and the accumulation region 14 are followed by a second region 16, whichis also part of the ion mobility separator 10 and may operate in an ionmobility separation mode.

During an ion mobility separation cycle, the ion mobility separationregion may be defined as that part of the ion mobility separator that iscausing ions to separate according to their ion mobility. As explainedin more detail below, the second region 16 may always form part of theion mobility separation region. The first region 12 may form part of theion mobility separation region during a first part of the ion mobilityseparation cycle, and may then form a trapping region during a secondpart of the ion mobility separation cycle.

To illustrate this, and referring to FIG. 2, time period T₀-T₁ mayrepresent a first time period of the ion mobility separation cycle. Ionsmay be transferred into the first region 12 (from accumulation region14) and may accumulate within the first region 12 to form a firstpopulation of ions.

During the first time period T₀-T₁ the accumulation region 14 may act inan ion guiding mode, and ions (e.g., from a continuous beam of ions) maybe urged through the accumulation region 14 and into the first region 12of the ion mobility separator 10, for example using a DC voltage, or DCtraveling wave. During this period the first region 12 may be in an iontrapping or accumulating mode, such that a first population of ions isaccumulated in the first region 12.

During a second time period T₁-T₂ (e.g., at time T₁) the accumulationregion 14 may switch from an ion guiding mode to an ion trapping oraccumulating mode, and ions within and entering the accumulation region14 may be prevented from entering the first region 12, such that asecond population of ions may start to be accumulated in theaccumulation region 14.

During the second time period T₁-T₂ (e.g., at time T₁) the first region12 may be switched from an ion trapping or accumulating mode to an ionmobility separation mode. In the second time period T₁-T₂ the firstpopulation of ions that has been accumulated in the first region 12 maythen be caused to separate according to their ion mobility within thefirst region 12 and second region 16, which may also be operating in anion mobility separation mode. Thus, during the second time period T₁-T₂the first region 12 and the second region 16 may form the ion mobilityseparation region.

A driving force, for example a DC voltage, or DC traveling wave may beapplied to the first region 12 and the second region 16, which may causethe first population of ions to separate according to their ion mobilityas they are driven through the ion mobility separation region.

At time T₂, prior to all ions in the first population of ions exitingthe second region 16, but after all ions from the first ion populationof ions have exited the first region 12, the first region 12 may beswitched from an ion mobility separation mode to an ion trapping oraccumulating mode. For example, the driving force may be removed fromthe first region 12. At this point only the second region 16 may formthe ion mobility separation region.

During a third time period T₂-T₃, the accumulation region 14 may beswitched from an ion trapping or accumulating mode to an ion guidingmode. During the third time period the second population of ions (whichhas accumulated within the accumulation region 14 during time periodT₁-T₂) is transferred and accumulated within the first region 12.

Ions entering and passing through the accumulation region 14 during timeperiod T₂-T₃, may also be transferred into and accumulate within thefirst region 12 to add to the second population of ions.

At the end of the third time period T₂-T₃ the second population of ionshas been accumulated in the first region 12. At this point theconditions of the second time period (T₁-T₂) can be repeated, in whichthe accumulation region 14 switches from an ion guiding mode to an iontrapping or accumulating mode, and the first region 12 switches from anion trapping or accumulating mode to an ion mobility separation mode.

As such, the second population of ions can be separated according totheir ion mobility in the first region 12 and the second region 16 in asimilar manner to the first population of ions during time period T₁-T₂.The first population of ions may not have fully exited the second region16, but the time periods may be chosen to avoid any overlap of the firstand second populations of ions. At this point a third population of ionscan be accumulated in the accumulation region 14, until the secondpopulation of ions have been driven from the first region 12 and theprocess can repeat for the third and further populations of ions.

The first region 12 may be significantly shorter in length compared tothe second region 16, in which case the second time period T₁-T₂ may bea small percentage of the ion mobility separation cycle (in this caseT₁-T₃). As such, transfer of the population of ions from theaccumulation region 14 to the first region 12 (during the second timeperiod T₂-T₃) can occur over a relatively long period of time comparedto the rapid ion transfer in the prior art. This can avoid ion heatingand ion losses associated with the prior art.

It will be appreciated that using the above method successivepopulations of ions can be separated according to their ion mobility. Itwill also be appreciated that the disclosed arrangement can lead to aduty cycle of 100%, since a beam of ions can be continuously passed intothe upstream accumulation region, which either accumulates the ions(e.g., during time period T₁-T₂) or passes them through to the firstregion 12 (e.g., during time period T₂-T₃). This can mean that no ionsfrom the incoming beam of ions are lost to the system or otherwise.

It will also be appreciated that the disclosed arrangement can avoid afast transfer of ions from an external ion trap into an ion mobilityseparator, which has been found to lead to ion heating and undesiredfragmentation. That is, the transfer of ions from the upstreamaccumulation region 14 into the first region 12 may be relatively slow,since the first region 12 may act in both a trapping mode and an ionmobility separation mode. This is in contrast to conventionalarrangements in which the transfer of ions from an upstream ion trapinto an ion mobility separator must be relatively quick, to avoidsubstantial dispersion of the ions in the ion mobility separator.

In accordance with the disclosure, a subsequent population of ions maybe accumulated in the upstream trapping region (or accumulation region14) whilst the preceding population of ions are separated according totheir ion mobility in the downstream trapping/separation region (orfirst region 12).

Once the preceding population of ions has exited the downstreamtrapping/separation region (or first region 12), but before thepreceding population of ions has exited the ion mobility separator 10(e.g., whilst the preceding population of ions is in the second region16), the subsequent population of ions can be transferred to thedownstream trapping/separation region (or first region 12). Thesubsequent population of ions may include ions that enter and are passedthrough the accumulation region 14 during the transfer.

After the preceding population of ions has left the ion mobilityseparator 10 (or once the subsequent population of ions has filled thedownstream trapping/separation region) the subsequent population of ionsbecomes the preceding population of ions and may be separated accordingto their ion mobility in the downstream trapping/separation region.

A (new) subsequent population of ions may be accumulated in the upstreamtrapping region (or accumulation region 14) whilst the precedingpopulation of ions are separated according to their ion mobility in thedownstream trapping/separation region (or first region 12), and thisprocess may repeat for successive populations of ions.

FIG. 3 shows schematically an ion mobility separator 10 in accordancewith the present disclosure.

The ion mobility separator 10 may comprise a plurality of electrodepairs 18. The electrode pairs 18 may be stacked adjacent to one another,and alternate phases of an AC or RF voltage may be applied to adjacentpairs of electrodes 18 in order to confine ions radially within the ionmobility separator 10 in the y direction. Each of the pairs ofelectrodes 18 may comprise a first plate electrode located opposite asecond plate electrode, and in use ions may travel through the gapbetween the first and second plate electrodes.

The successive pairs of electrodes 18 may form a first array of firstelectrodes located opposite a second array of second electrodes. Thefirst electrodes and/or the second electrodes may form plate, planar ormesh electrodes. The plane of the first and/or second electrodes may beoriented in the y direction as shown in FIG. 3, or the z or x direction(or any other direction suitable to confine ions within the gap betweenthe first and second electrodes).

As discussed above a population of ions 20 may be confined in the ydirection by application of an AC or RF voltage to the pairs ofelectrodes 18, wherein opposite phases of the AC or RF voltage may beapplied to adjacent pairs of electrodes 18.

Ion confinement in the x direction may be accomplished by creating a DCconfining field in the x direction. The DC confinement can beaccomplished by providing separate DC electrodes (not shown), forexample intermediate plate, planer or mesh electrodes that runlongitudinally along the length of the ion mobility separator 10, andbetween the first array of first electrodes and the second array ofsecond electrodes.

In an alternative embodiment, the ion mobility separator may comprise aplurality of electrodes, wherein each electrode has an aperture throughwhich ions travel in use. In such an embodiment, alternate phases of anAC or RF voltage may be applied to adjacent electrodes to confine ionsradially within the ion mobility separator.

The population of ions 20 may be confined in the z direction(longitudinally) by application of appropriate DC potential to adjacentpairs of electrodes 18. For example, a potential well may be created byapplying appropriate DC potentials to the successive pairs of electrodes18. For example, an electrode pair may be held at a relatively low DCpotential compared to the electrode pairs either side of it. This cancreate a DC potential well that confines ions within the well in the zdirection.

FIG. 3 shows schematically the position of a population of ions in thefirst region 12 and the accumulation region 14 over the time from T₀-T₃as described in relation to FIG. 2.

In the first time period T₀-T₁, it can be seen that the ion populationin the first region 12 has a small spatial spread in the z directioncompared to the spatial spread in the x direction. As the first region12 may form part of the ion mobility separation region of the ionmobility separator (e.g., during the second time period T₁-T₂), it canbe advantageous that, prior to the first region 12 switching suddenlyfrom an ion trapping mode to an ion mobility separation mode, ions arefocused within the first region 12 with a small spatial spread in thedirection of ion mobility separation (i.e., the z direction). This canensure maximum ion mobility resolution without requiring additionalcompression of the ion cloud in this direction, since the ions that areto be separated according to their mobility have a very similar startingpoint.

Furthermore, ions may be allowed to have a larger spatial spread in adirection orthogonal to the direction of ion mobility separation (i.e.,the x direction). This can maximise the space charge capacity of thefirst region 12, which can switch suddenly from a trapping region to anion mobility separation region, as well as the downstream ion mobilityseparation region (i.e., the second region 16).

Ions may be driven or transferred between the first region 12 and/or theaccumulation region 14 and/or the second region 16 using static DCfields, DC potentials or travelling DC waves, any combination of thesedriving methods, or any other suitable driving method.

The accumulation region 14 may be extended in the z direction and/or thex direction. As the time scale over which ions are transferred from theaccumulation region 14 to the first region 12 (e.g., the time periodT₂-T₃) is extended using the method described herein, ions may still betransferred with minimal losses even if the accumulation region 14 has arelatively large extent in the z direction.

Both trapping regions (i.e., first region 12 and accumulation region 14)may be extended in either or both the x and y directions. In the ionmobility separator 10, and in particular when applying a transient DCpotential to successive pairs of electrodes in order to urge or (i.e., atravelling wave), the distance between the confining pairs of electrodes18 in either the x or y direction may be limited, such that sufficientdriving force is experienced by ions without having to apply anexcessively large transient DC potential.

The first region 12 and/or the accumulation region 14 and/or the ionmobility separator 10 may be extended in this manner to form an annularvolume to maximize space charge capacity.

From the above discussion it will be appreciated that accumulating asubsequent population of ions within the first region 12 whilst thepreceding population of ions are being separated according to their ionmobility within the ion mobility separator 10 can achieve betterresolution and performance compared to conventional methods, for examplethose that transfer ions into an ion mobility separator from an externalion trap.

Various embodiments are contemplated and defined herein that improvemethods of ion mobility spectrometry. For example, ensuring that the ionpopulation in the first region 12 has a relatively small spatial spreadin the direction of ion mobility separation, and a relatively largespatial spread in the direction orthogonal to ion mobility separation,can achieve improved resolution and performance as discussed above.

Using two trapping regions (i.e. the first region 12 and theaccumulation region 14) as described herein can eliminate therequirement for rapid ion transfer from a separate ion trapping regionto an ion mobility separation region whilst maintaining high duty cycle.Extending the ion transfer time using two trapping regions as discussedherein may reduce ion losses and unwanted ion fragmentation.

The ion mobility separator referred to herein (such as ion mobilityseparator 10) may be part of a larger device, such as a mass or ionmobility spectrometer, or a hybrid ion mobility-mass spectrometry(IMS-MS) or mass spectrometry-ion mobility (MS-IMS) device. All regionsof (or the entirety of) the ion mobility separator may be arranged andadapted to separate ions according to their ion mobility, for example inat least one mode of operation. Using such a definition, theaccumulation region 14 would not form part of the ion mobility separator10, even if it formed part of the same physical device, for example astack of electrodes. All regions of (or the entirety of) the ionmobility separator may be operated at the same or substantially the samepressure. All regions of (or the entirety of) the ion mobility separatormay contain the same buffer gas.

Various embodiments are contemplated in which the arrangement of first,second and second regions may be applied to any type of ion separation,for example mass or mass to charge ratio. For example the ion mobilityseparator 10 may instead be a mass separation device, for example ahigh-pressure mass separation device. The features described above inrelation to the ion mobility separator, for example the arrangement offirst, second and second regions and/or the electrodes and/or thevoltages applied thereto, may also be features of the mass separationdevice. The method features relating to the first, second and secondregions may be applied mutatis mutandis to a mass separation devicecomprising these regions and arranged in the same manner as the ionmobility separator described above. For example, in such embodiments thefirst region may be arranged and adapted to switch between a trappingmode, in which ions are substantially trapped within the first region,and a separation mode, in which ions are separated according to adifferent physico-chemical property, for example mass or mass to chargeratio, within the first region.

Although the present invention has been described with reference topreferred embodiments, it will be understood by those skilled in the artthat various changes in form and detail may be made without departingfrom the scope of the invention as set forth in the accompanying claims.

The invention claimed is:
 1. A method of separating ions according totheir ion mobility, comprising: (i) accumulating a first population ofions in a first region of an ion mobility separator; (ii) separatingsaid first population of ions according to their ion mobility in saidfirst region of said ion mobility separator; and (iii) accumulating asecond population of ions in said first region of said ion mobilityseparator whilst said first population of ions are being separatedaccording to their ion mobility in said ion mobility separator, whereinsaid ion mobility separator comprises a second region downstream of saidfirst region, and step (ii) comprises separating said first populationof ions according to their ion mobility in said first and second regionsof said ion mobility separator; and wherein said first region of saidion mobility separator switches between a trapping mode, in which thefirst region is configured such that ions are substantially trappedwithin said first region and not caused to separate according to theirion mobility, and an ion mobility separation mode, in which the firstregion is configured such that ions are separated according to their ionmobility within said first region.
 2. A method as claimed in claim 1,wherein step (ii) comprises accumulating said second population of ionsin an accumulation region upstream of said first region, whilst saidfirst population of ions are being separated according to their ionmobility in said first and second regions of said ion mobilityseparator.
 3. A method as claimed in claim 2, wherein step (iii)comprises transferring said second population of ions from said upstreamaccumulation region and into said first region, and continuing to passions through said upstream accumulation region and into said firstregion so as to accumulate said second population of ions in said firstregion.
 4. A method as claimed in claim 3, wherein step (iii) comprisesaccumulating said second population of ions in said first region of saidion mobility separator whilst said first population of ions are beingseparated according to their ion mobility in said second region of saidion mobility separator, and after said first population of ions haveexited said first region of said ion mobility separator.
 5. A method asclaimed in claim 1, further comprising: (iv) separating said secondpopulation of ions according to their ion mobility in said ion mobilityseparator; and (v) accumulating a third population of ions in said firstregion of said ion mobility separator whilst said second population ofions are being separated according to their ion mobility.
 6. A method asclaimed in claim 5, further comprising: repeating steps (ii) to (v) forsaid third and further populations of ions, such that subsequentpopulations of ions are accumulated in said first region of said ionmobility separator whilst preceding populations of ions are beingseparated according to their ion mobility.
 7. A method as claimed inclaim 1, further comprising directing a beam of ions continuously intothe upstream accumulation region.
 8. A method as claimed in claim 1,wherein said ion mobility separator comprises an RF-confined ionmobility separator.
 9. A method as claimed in claim 8, wherein said ionmobility separator comprises a plurality of electrodes stacked adjacentto one another, wherein alternate phases of an RF voltage are applied toadjacent electrodes.
 10. A method as claimed in claim 9, wherein each ofsaid plurality of electrodes comprise apertures through which ionstravel in use.
 11. A method as claimed in claim 9, wherein saidplurality of electrodes comprises a plurality of pairs of electrodesstacked adjacent to one another, wherein alternate phases of an RFvoltage are applied to adjacent pairs of electrodes.
 12. A method asclaimed in claim 11, wherein each of said pair of electrodes comprises afirst plate electrode opposite a second plate electrode, wherein in useions travel through the gap between said first and second plateelectrodes.
 13. A method as claimed in claim 2, wherein said firstregion, said second region, and said accumulation region compriseRF-confined regions.
 14. A method as claimed in claim 1, wherein step(i) comprises accumulating said first population of ions in said firstregion when in the trapping mode such that within the first region thefirst population of ions are not caused to separate according to theirion mobility and has a relatively small spatial spread in the intendeddirection of ion mobility separation during the ion mobility separationmode, and a relatively large spatial spread in the direction orthogonalto the intended direction of ion mobility separation during the ionmobility separation mode.
 15. A method as claimed in claim 2, whereinsaid accumulation region of said ion mobility separator is arranged andadapted to trap or accumulate a population of ions and then transfer thepopulation of ions from the accumulation region into the first region.16. An ion mobility spectrometer or separation device comprising: an ionmobility separator comprising a first region arranged and adapted toaccumulate consecutive populations of ions, and a second regiondownstream of said first region; and a control system arranged andadapted: (i) to accumulate a first population of ions in said firstregion of said ion mobility separator; (ii) to separate said firstpopulation of ions according to their ion mobility in said first regionof said ion mobility separator; and (iii) to accumulate a secondpopulation of ions in said first region of said ion mobility separatorwhilst said first population of ions are being separated according totheir ion mobility in said ion mobility separator, wherein, in step(ii), said control system is arranged and adapted to separate said firstpopulation of ions according to their ion mobility in said first andsecond regions of said ion mobility separator; wherein said first regionof said ion mobility separator switches between a trapping mode, inwhich the first region is configured such that ions are substantiallytrapped within said first region and not caused to separate according totheir ion mobility, and an ion mobility separation mode, in which thefirst region is configured such that ions are separated according totheir ion mobility within said first region.
 17. An ion mobilityspectrometer or separation device as claimed in claim 16, furthercomprising an accumulation region upstream of said first region, whereinsaid control system is arranged and adapted: to accumulate said secondpopulation of ions in said accumulation region whilst said firstpopulation of ions are being separated according to their ion mobilityin said first and second regions of said ion mobility separator.
 18. Amethod of separating ions in a device, wherein the device comprises afirst region, an accumulation region upstream of the first region and asecond region downstream of the first region, wherein said first region,said second region, and said accumulation region comprise RF-confinedregions, and the method comprises the steps of: (i) driving a firstpopulation of ions through the first and second regions so as toseparate a first population of ions according to their ion mobility,whilst accumulating a second population of ions in the accumulationregion; (ii) after the first population of ions has exited the firstregion, but prior to the first population of ions exiting the secondregion, transferring the second population of ions into the first regionand trapping the second population of ions within the first regionwithout causing the second population of ions to separate according totheir ion mobility; and then (iii) driving the second population of ionsthrough the first and second regions so as to separate the secondpopulation of ions according to their ion mobility, whilst accumulatinga third population of ions in the accumulation region; (iv) repeatingsteps (ii) and (iii) for the third and further populations of ions so asto successively separate the third and further populations of ions; andwherein said first region of said ion mobility separator switchesbetween a trapping mode, in which the first region is configured suchthat ions are substantially trapped within said first region and notcaused to separate according to their ion mobility, and an ion mobilityseparation mode, in which the first region is configured such that ionsare separated according to their ion mobility within said first region.19. A method as claimed in claim 18, wherein: step (ii) comprisesaccumulating said second population of ions in said first region when inthe trapping mode such that within the first region the secondpopulation of ions are not caused to separate according to their ionmobility and has a relatively small spatial spread in the intendeddirection of ion mobility separation during the ion mobility separationmode, and a relatively large spatial spread in the direction orthogonalto the intended direction of ion mobility separation during the ionmobility separation mode.